In recent years, CO2 emissions have been strictly regulated due to increasing environmental concerns. In the field of automobiles, weight reduction of car bodies and improvements in fuel efficiency have emerged as major challenges. Accordingly, automobile parts have become increasingly thinner by the increasing use of high-strength steel sheets. In particular, high-strength steel sheets having a tensile strength (TS) of 980 MPa or higher are now being increasingly used in automobile parts.
High-strength steel sheets used in automobile parts such as structural parts and reinforcement parts of automobiles are required to have excellent formability. In particular, high-strength steel sheets for use in parts having complicated shapes are required to excel in not only one of but both elongation and stretch flangeability (also referred to as hole expandability). Automobile parts such as structural parts and reinforcement parts described above are also required to have excellent impact energy absorption capability. To improve the impact energy absorption capability, it is effective to increase the yield ratio of the steel sheet used. Automobile parts that use steel sheets having high yield ratios can absorb impact energy efficiently at low deformation. The yield ratio (YR) discussed here is a value of the ratio of the yield stress (YS) to the tensile strength (TS) and is expressed as YR=YS/TS.
Heretofore, dual phase steels (DP steels) having a ferrite-martensite structure have been known as high-strength thin steel sheets that have both high strength and formability. An example of steel sheets having high strength and excellent ductility is TRIP steel sheets that use transformation induced plasticity of retained austenite. TRIP steel sheets have a steel sheet structure containing retained austenite. When TRIP steel sheets are worked and deformed at a temperature equal to or higher than a martensite transformation start temperature, retained austenite is induced to transform into martensite by stress and a large elongation is obtained. However, TRIP steel sheets have a problem in that transformation of retained austenite into martensite during a punching process causes cracks to occur at the interfaces with ferrite and degrades hole expandability (stretch flangeability).
An example of a steel sheet having stretch flangeability improved from the TRIP steel sheets is described in, for example, Japanese Unexamined Patent Application Publication No. 2005-240178 which discloses a high-strength cold rolled steel sheet having excellent elongation and stretch flangeability and a steel structure that satisfies the following: retained austenite: at least 5%, bainitic ferrite: at least 60%, and polygonal ferrite: 20% or less (including 0%). Japanese Unexamined Patent Application Publication No. 2002-302734 discloses a high-strength steel sheet having excellent elongation and stretch flangeability, the steel sheet containing 50% or more of tempered martensite as a base structure in terms of occupation ratio in the entire structure, and 3% to 20% of retained austenite as a second phase structure in terms of occupation ratio in the entire structure.
Generally speaking, DP steels have low yield ratios since mobile dislocations are introduced into ferrite during martensite transformation and thus have low impact energy absorption capability. A steel sheet of Japanese Unexamined Patent Application Publication No. 2005-240178, which is a TRIP steel sheet that makes use of retained austenite, has insufficient elongation relative to strength and it is difficult to obtain sufficient elongation in a high-strength region where TS is 980 MPa or higher. According to Japanese Unexamined Patent Application Publication No. 2002-302734, steel sheets described as having excellent elongation and stretch flangeability specifically disclosed in Examples have low yield ratios, and TS thereof is at the 590 to 940 MPa level. Thus, those steel sheets do not have excellent elongation and stretch flangeability in a high strength region of 980 MPa or higher, and high yield ratios.
As discussed above, it is difficult for a high-strength steel sheet having a tensile strength of 980 MPa or higher to have a high yield ratio to maintain excellent impact energy absorption capability, and assure elongation and stretch flangeability to maintain excellent formability. A steel sheet having all these properties is desirable.
It could therefore be helpful to provide a high-yield-ratio, high-strength cold rolled steel sheet having excellent elongation and stretch flangeability and a production method therefor.